1. Field of the Invention
The present invention relates to a cathode-ray tube (CRT), an electron gun to be employed in the CRT, a beam control electrode included in the electron gun, and a method of fabricating the beam control electrode. More specifically, the present invention relates to an electrode plate having thin portions provided with beam passage holes and excess metal relieving slots for relieving excess metal so that excess metal may not form protrusions when forming the thin portion, used as a beam control electrode, and enabling the accurate disposition of an adjacent electrode at a predetermined distance therefrom to form beams of a desired shape. The shape of the beam in the periphery of a screen can automatically be corrected by the beam control electrode.
2. Description of the Related Art
An electron beam emitted by an electron gun is deflected for the two-dimensional scanning of a fluorescent body to form a desired picture on a screen. The electron beam emitted by the electron gun has a circular cross section and forms a substantially circular spot at the center of the screen as shown in FIG. 5A. However, the electron beam is deformed by electromagnetic deflection and forms horizontally elongate spots in the corners of the screen, namely, the periphery of the screen, as shown in FIG. 5A. A picture formed by such distorted spots of the electron beam is distorted.
A method proposed to correct the distortion of a picture uses an electron beam which forms, for example, a vertically elongate spot at the center of the screen as shown in FIG. 5B. This electron beam, which forms a vertically elongate spot, i.e., a distorted spot, forms substantially circular spots in the corners of the screen as shown in FIG. 5C when subjected to electromagnetic deflection. FIG. 6 shows a conventional color electron gun for a three-gun three-beam type electron gun (color tube). This electron gun emits electron beams of an intentionally deformed cross section. The electron gun is provided with three in-line cathodes KR, KG and KB.
A plurality of plate-shaped or cylindrical grids G1 to G6, i.e., beam control electrodes, are arranged at predetermined intervals in the direction of travel of beams for the cathodes KR, KG and KB. The fifth grid G5 and the sixth grid G6 form a main electron lens (convex lens).
FIG. 7 shows the electron gun in a sectional view. In FIG. 7, the cathode KR is disposed at the center, an electron beam EB deflected by the main electron lens ML reaches a screen 12.
The shapes of spots formed by an electron beam in the corners of the screen can be corrected by shaping the cross section of the beam in a vertically elongate shape, i.e., a shape elongate in the vertical scanning direction, by controlling beam divergence angle .theta..
Beam divergence angle .theta. can be controlled mainly by the shape of the second grid G2. The second grid G2 is formed in a structure shown in FIG. 8 to control beam divergence angle .theta..
Generally, beam passage holes 22R, 22G and 22B are formed in thin portions 20R, 20G and 20B, respectively, of the second grid G2 to secure necessary strength for the second grid G2 and to secure a desired divergence angle .theta.. Formation of such thin portions in an electrode is called coining. As shown in FIG. 9, the thin portions 20R, 20G and 20B formed by coining have a horizontal length, i.e., length along an axis X-X', greater than a vertical length, i.e., length along an axis Y-Y'. The beam passage holes 22R, 22G and 22B are formed at the centers of the thin portions 20R, 20G and 20B, respectively. FIG. 10 is a sectional view taken on the axis X-X' in FIG. 9.
Since the thin portions have an asymmetrical shape (astigmatic coining), portions of the second grid G2 on the side of the vertical scanning direction are thick and a converging lens having a high converging ability is formed, because an electric field is created in the beam passage holes. The converging lens having a high converging ability reduces the divergence angle of the beam.
Since portions of the second grid G2 on the side of the horizontal scanning direction are thin, a converging lens having a low converging ability is formed and hence the divergence angle of the beam is large. Therefore, the beam EB travels through the central portion of the main electron lens ML having a small curvature and a low converging ability with respect to the vertical scanning direction, and the beam EB travels through an outer portion of the main electron lens ML, having a large curvature and a high converging ability with respect to the horizontal scanning direction. Consequently, the beam is converged greatly with respect to the horizontal scanning direction and the beam forms a vertically elongate spot.
Usually, the second grid G2 having the thin portions formed by coining is manufactured by the following process.
FIG. 11 shows only a portion for an R beam of a metal plate used as the second grid. A prepared hole 26R is formed at a predetermined position in a metal plate 18 by punching as shown in FIG. 11A. The prepared hole 26R is formed to relieve excess metal during press working. The metal plate 18 is subjected to press working for coining using a punch 28 as shown in FIG. 11B to form a rectangular thin portion 20R as shown in FIG. 11C.
The prepared hole 26R serves as an excess metal relieving slot during press working and its diameter is reduced as shown in FIG. 11D. A portion of the metal plate 18 having the prepared hole 26R is punched again with a punch 30 to form the beam passage hole 22R of a predetermined diameter as shown in FIGS. 11D and 11E. Thus, the second grid G2 having the thin portion 20R of predetermined dimensions as shown in FIG. 11E is completed. Other thin portions 20G and 20B are formed by the same process and hence the description of processes for forming the thin portions 20G and 20B will be omitted.
It is possible that excess metal is collected in part of the thick portion of the metal plate 18 in a bulge when the forming the thin portion 20R by press working using the punch when forming the second grid G2 by steps shown in FIG. 11. This excess metal cannot be relieved only by the prepared hole 26R, and a protrusion 24 is formed in a portion of the thick portion around the thin portion 20R as shown in FIG. 11F. The formation of the protrusion 24 remarkable when the thin portion 20R must be formed in a very small thickness.
If the protrusion 24 is formed around the thin portion 20R, it is possible that the second grid G2 and the third grid G3 cannot be attached to beadings 14 and 16 (FIG. 7) with the second grid G2 and the third grid G3 spaced a predetermined distance apart. The plurality of grids G1 to G6 are held on the beadings (glass) 14 and 16 at predetermined intervals.
Since the space between the second grid G2 and the third grid G3 is very narrow, the second grid G2 and the third grid G3 are spaced by a spacer 34 as shown in FIG. 10 for a beading process.
If the protrusion 24 of an indefinite height is formed by the coining process, the insertion of the spacer 34 between the second grid G2 and the third grid G3 is obstructed by the protrusion 24 and the second grid G2 and the third grid G3 cannot be held with a design interval therebetween. Consequently, an electron gun having a desired ability cannot be constructed.